A number of semiconductor manufacturing processes involve processing of semiconductor materials at pressures below atmospheric pressure. For example, processes such as sputtering, evaporation, reactive ion etching (RIE), molecular beam epitaxy (MBE), chemical vapor deposition (CVD), and others may be advantageously performed in a vacuum chamber in which the pressure may be reduced to less than atmospheric pressure. As is known in the art, performing semiconductor processing in a vacuum chamber may provide an environment in which the mean free path (MFP) of particles in the system is large relative to the dimensions of the system and the materials being processed.
Semiconductor devices are typically fabricated on a substrate that provides mechanical support for the device and often participates in the electrical operation of the device as well. Silicon, germanium, gallium arsenide, gallium nitride, sapphire and silicon carbide are some of the materials commonly used as substrates for semiconductor devices. Many other materials, including semiconductor as well as non-semiconductor materials, may also be used as substrates for semiconductor devices. Semiconductor device manufacturing typically involves fabrication of many semiconductor devices on a single substrate.
Substrates are typically formed in the shape of circular wafers having a diameter ranging, for example, from less than 1 inch (about 25 mm) to over 12 inches (about 300 mm) depending on the type of material involved. Other shapes such as, for example, square, rectangular or triangular wafers are possible, however. Semiconductor devices are formed on the wafers by the precise formation of thin layers of semiconductor, insulator and metal materials that are deposited and patterned to form useful semiconductor devices such as diodes, transistors, solar cells and other devices.
In some cases, it may be desirable to maintain the substrate at or below a predetermined temperature during vacuum processing, as the temperature of the substrate may affect the processing that is being performed. For example, many properties of materials and processes may be affected by temperature, including reaction rates and diffusion rates, which typically follow a temperature-dependent Arrhenius relationship based on an activation energy constant. In other cases, it may be desirable to keep certain types of materials under a given temperature to avoid damage to sensitive materials and/or films. For example, photoresist and/or other polymeric materials may undesirably reflow at elevated temperatures.
Accordingly, it is known to cool substrates during vacuum processing. In typical cooling methods, a substrate is affixed to a heat sink which may be actively and/or passively cooled. Active cooling methods include, for example, fluid cooling circuits using water, helium gas, glycol, or another fluid with appropriate heat transfer characteristics. Passive cooling methods may employ a static thermal mass (e.g., a large piece of thermally conductive material such as copper) as the heat sink. Hybrid cooling methods may include, for example, a refrigerated static thermal mass for heat removal.
A conventional vacuum processing system is illustrated in FIG. 1. As shown therein, a conventional vacuum processing system may include a vacuum chamber 10 in which a number of substrates 12 are positioned. The substrates 12 may include, for example, silicon, germanium, gallium arsenide, sapphire and/or silicon carbide. The substrates 12 may include non-semiconductor materials such as metals and/or ceramics. The substrates 12 are positioned on a heat sink 14 which is supported by a submount 16. As discussed above, the heat sink 14 may be actively and/or passively cooled. The vacuum chamber 10 may include other features (not shown) for processing the substrates 12, such as, for example, an MBE source, input and exhaust gas lines, evaporation sources, sputter targets, plasma-generating electrodes, and/or other features. In some cases, the vacuum chamber 10 may be loadlocked. Thus, there may be no additional lines running from the heat sink 14 to a location external to the vacuum chamber 10.
Typically, the substrates 12 are affixed to the heat sink 14 by means of mechanical clamping, electrostatic clamping or some other clamping method. Physical clamping mechanisms may be undesirable, because they may break or otherwise damage delicate semiconductor wafers and/or films. Thus, in some systems, gravity may be used to hold the substrates 12 in place on the heat sink 14. However, it may be difficult to make uniform contact between the substrates 12 and the heat sink 14 using only gravity. Without adequate contact between the substrates 12 and the heat sink 14, hot spots may form in the substrates 12 which may adversely affect the substrates 12 and/or the processing of the substrates 12.